It is well known to provide a centrifugal compressor, which operates at high speeds, for use in the transport and energy industries.
Centrifugal compressors offer advantages over positive displacement compressors, such as reciprocating compressors, rotary screw compressors, and rotary vane compressors, in that they have a much higher power density, offer oil-free compression, a low parts count and steady flow delivery.
One shortcoming of centrifugal compressors when compared with positive displacement compressors is that the shaft speed must increase as the flow rate reduces. An increased shaft speed will result in increased wear and tear on bearings, reducing the life of the compressor. Centrifugal compressors also generally exhibit low efficiency at low flow rates. For these reasons, positive displacement compressors have been the primary method by which low flow rate applications have been managed thus far, with their disadvantages of noisy operation and high maintenance costs.
Some existing centrifugal compressors run at 60,000 to 250,000 rpm in order to provide the required performance.
Centrifugal compressors have rotors which are either forward-swept or backward-swept. A forward-swept design, in which the rotors have blades that are curved towards the direction of flow, is beneficial because there is no need to increase the static pressure in the rotor, which solves various problems which can otherwise result from a pressure gradient between the rotor inlet and outlet. A centrifugal compressor rotor with an exaggerated forward-sweep, between 20 and 90 degrees to a radius of the compressor, is particularly useful because the speed of the fluid at the rotor exit is greater than the blade speed, providing for a large pressure rise from a relatively small compressor, i.e., a small diameter rotor. A forward-swept tangential (as opposed to radial) centrifugal compressor of this type is disclosed in WO2005024242. However, forward-swept compressors are known to be inherently unstable.
It is an object of the invention to provide an improved diffuser for a centrifugal compressor, allowing more stable operation at low flow rates without excessive shaft speed.